When the engine is controlled by a programmed CPU, an input circuit for holding information from sensors which detect engine conditions, in the form capable of being transmitted to the CPU in response to a request from the CPU, and an output circuit for converting digital signals from the CPU to a pulse signal for driving an engine control mechanism are required. Those input and output circuits must be fabricated in the form of a highly integrated circuit.
However, since the engine controlling circuits are mounted on a vehicle, the surrounding temperature changes significantly depending on the conditions under which the vehicle is operated. As a result, the circuits must be designed taking the worst condition into consideration and hence a high integration density has been difficult because of thermal requirements. For example, the vehicles may be used in a tropical zone or they may be used in broiling weather. Taking those conditions into consideration, the surrounding temperature may rise to a very high temperature (for example, 100.degree. C. while taking the influence of radiation heat of the engine into consideration). It is necessary to avoid the heat concentration so that the junction temperature of a circuit component is kept below a prescribed temperature even under those conditions. For this reason, the integration density could not be increased.
In order to resolve the above problem, it is necessary to construct the input and output circuits by a circuit configuration having circuit components having a small amount of heat generation.